Automatic gain control circuits



sept. 22, 1936. .w. R. KOCH AUTOMATIC GAIN CONTROL CIRCUITS Filed March la', 1955 WINFIELD R. KOCH ATTORNEY Patented Sept. 22,

fUNlTl-:D STATE Winfield R. Koch, Camden, N. J., vassignor to Radio Corporation of America, a'corporation of Delaware Application March y18, 193,3,7Slerial No. '661,496"

s Claims. (C1, 25o- 20) v s GFFICE My present invention relates to high frequency freceivers, and more particularly to automatic volume control arrangements for such receivers utilizing a control tube of the thyratron type.

It has previously been proposed to provide radio rreceivers, with automatic volume control devices whereby the ultimate sound output is rendered substantially independent vof fading and other phenomena which tend to'cause the amplitude of an incoming signal to greatly vary. In the operation of receivers equipped with such automatic volume control devices, however, some dissatisfaction has been experienced by reason of the 'factthat such d-evices did not hold the signal strength sufliciently close to a constant value at the grid of the receiver detector. Y f Accordingly, it may be stated that'it is one of the main objects of my present invention to provide .an improved automatic volume control arrangement for a radio receiver, the arrangement functioning to maintain the signal strength lsubstantially constant at the grid of the detector, the automatic volume control circuit employing a control tube of the thyratron type.

Another important object of the present invention may be stated to involve the provision of an automatic volume control tube for an 'automatic gain control network, adapted for use in a radio receiver, wherein the control tube is a grid c'ontrolled rectifier, the function of the grid being to; ,control the starting of the anode current, and an lalternatingvoltage being applied to the anode.

And still other objects of the present :invention are to improve generally the eiciencyrof `gain control circuits for radio receivers, and to especially provide an automatic volume control network for a radio receiver which is not only ureliable in operation but economically manufactured and assembled.

w The novel features which I believe to` be characteristic of my invention are set forth in particularity in the appended claims, the invention itself, however, as to both its organization and method of operation will best be understood by 'reference 4to the following description taken in `connection with the drawing vin which I have indicated diagrammatically several circuit organizations whereby my invention may be v carried into effect. 5 .'In'the drawing, 1 i i 1 Fig;1 diagrammatically shows a radio receiver embodying a control tube according to the present invention,

`FiggZ shows a modification of the circuit shown in Fig. 1.

present time.

B throughrthe primary winding '3. 1,transformer M couples theanode circuit of the tube Ito the input electrodes of the rectier Referringnow to the accompanying-drawing `wherein like yreference characters designate corresponding circuit elements in the different figures, there is shown in Fig. 1 in purely conventional form the elements of a radio receiver which `are essential to aniunderstan-ding of the present invention. The numeral l designates an ampliy ner tube, which may be any of the well known .radio frequency amplifier tubes employed at the The tube is shown used in a radio frequency amplifier stage. rThe resonant input circuit 2 of the tube has its high radio frequency potential side connected to the grid of the tube, Ywhile the low potential side is connected, through a capacitor C, to the negative leg of the cathode ofytube l., The anode of tube l is connected to the positive terminal of the anode potential source The coupling Atube 4. s Y

The resonant: input circuit 5 of the rectifier The the primary winding 'l and the resistor 8. The audio frequency coupling transformer M1 couples 'theianode circuit ofthe rectifier to any appropriate audio frequency network, the latter includinggif-'desiredLoneior more stages of audio frequency amplification, and a final reproducer.

fr The-.automatic volume control network comprises 'the tubeii of thethyratron type. In carrying outthepresentrinventioniit is preferred to `utilize a grid controlled rectifying device as now verywell-knownunder the registered trade-mark fThyratron- This rectifying device, which hereinafter will'be referred to for the sake of brevity :as:aithyratron,l is supplied with alternating potentialf `andthe grid thereof is normally so biased e that rectified current flows in the output circuit.

'Theanode of the thyratron tube 9 is supplied with an alternating potential, of for example 300 volts, .1 which potential .may conveniently be derived from the secondary winding It of a power trans- ,y

former; M2,v the primary winding l I being con- .znected .to a source of 50 cycle voltage. This transformer Mzfcould, also, be used to energize a rectifienmot shown) for supplying direct current potential forthe various cold electrodes of the tubes I and 4, and to supply heating current to the various cathode heating currents of the receiver.

The control electrode of the tube 9 is connected, through a lead I2, to one side of a resistor 8, While the other side of the resistor 8 is connected to the electrical midpoint of the cathode resistor I3 through a path which includes the biasing potential C1, `the negative terminal fof this :biasing potential source being connected tothe ,controlelectrode of the tube 9. The anode of tube 9 is connected to the electrical midpoint of the resistor l3 V through a path which includes the Ypower transformer Winding Ill and the volumecentrol\res'istor I4 in series. Y

rThe negative side of thefresistorlll,isbQnneCted to the low radiol fequency potential side of the input circuit 2 through a path, designated A. V, Cif,Y

which includes the filter resistorR. `"The positive terminal of the resistor I4 is connected vtofthe negative legV of the cathode of tube I through a path -comprising'the lead I5 and the ybiasing potential source 02, uthe positive terminal of this source being connected to the vpositivesideoifresistor 'I'II.

The plate current -of =thethyratren 2tube,'-with Valternating voltage on the `plate, -can be -con- 'trolled to la certain extent by `the sbias voltage.

rIhe mutual conductance A-under vthese vconditions Y 'is much better than the ordinaryelectron discharge-tube. Because of the extremely lhigh ymutual conductance of the thyratron tube, it may "beused in a radio receiver to automatically ad- `just 4the sensitivity of ythe Yreceiver soV as to keep the'sign-al level at the detector grid at Va substantially 'constant level. With alternating current vvoltage appliedv tothe plate of the'thyratron the plate current 'over'part of the lrangefdepends upon 4the `bias Voltage applied. Y

The function 'of the grid-of Ithe thyratron tube is to control lthe-starting ofthe plate current. -A negative grid potential, -of sufficient value with respect to the cathode, will prevent the occurrence of a discharge. Once the discharge has -started'the grid potentiahas usually applied, `does not control the anodecurrent. If an `alternating voltage is applied lto the anode, ythe'grid has van opportunity to *regain control during the negative half of each cycle, and to retain this control for as longapericd during-the'extent ofthe cycle-as its -potential -is sumciently negative. -Bythus controlling the timefof starting ofthe anode current `in each positive half cycle -the average anode current is controlled.

-An Aincrease -in signal strength -on the -grid o f vthe grid `leak detector tube V4-decreases vthe flowof anodecurrent inthe anodecircuitof the-detector. The potential drop across the resistor Y8 becomes less, Whichfreducesfthe bias=on the thyratron, and, laccordingly, `increases vthe thyratron plate current. There results an increase, therefore, in the 'potentialfdropacrossthe resistor I4,.and this iin- A'crease'in potential drop is employed tobias oifthe amplifier tube '-I soas to reduce its amplification. The reverseloperation takes place when the-sig- `nel :strength applied 'to 4the grid of the detector tube Il! -decreases below Ya predetermined signal level.

'The alternating .current ripple vin the voltage across the :resistor I4 .is fltered out Vby vthe re-Y sistor-RIandlcapacitorrC. "It is desirable thatthe Vthyratronlbe shielded and its leads ltered to keep -its-radiointerference con'ned. It should be noted uthat exchange of only a `Afewrtenths of Ya .voltongthe 'gri'dfof ithe :thyratron lis ample. 'JIt 'should vbe obfserved that the -thyratron :control ltube vis condai-rolled indirectly by Jthe detector ltube.

Of course, the present invention is not limited to a radio receiver employing but a single stage of radio frequency amplification preceding a grid leak type of detector. For example, the receiver may beV of the tuned radio frequency type or the superheterodyne type. In the case of the former, one or more stages of radio frequency amplificationcould be used aheaclof vthegricl leakdetector tube i, andin such ,case-,all the resonantiinput circuits of the various tubes would include variable tu-ning condensers having their rotors uni-controlled in the well known manner. In this case, also, .the fthyratron tube S could be used to control .the amplification of .all the radio frequency amlplifier tubes.

In Ythe :case -of :a ,'superheterodyne receiver, the tube I may be'consideredto represent the intermediate 'frequency amplifier, with its input circuit ilxedly resonant to the operating intermediate frequency. The rectier tube 4 would then be employed inLthe second-detector stage. Of course, in such a receiver the resonant circuit A2 VWould be coupled .to `the output of the iirst detector stage, and the thyratron tube S could .be used .to

-control the amplication of a radio frequency .amplifier aheaclof the firstdetector, as Well asthe the .control electrode of the thyratron tube .9

.islconnected to thegridcircuit of the rectier tube Such 30 II :through Va path which includes ,the lead 20 and .radio frequency coupling condenser 2 I.

The resistor P'haseneside thereof `.connected .to ,the anode oftubegi ,through the primary Winding ID ,of the'power transformer M2. The eleotrical midpoint of the cathode .resistor I3 gis `Qonnected to point ,12,2 v,of the ,resistor P, Ywhile the Agri-clfo the tube 5 is connected to Athe point 2,2 through azbias resistorrZrSand a bias source. The 4negativelsideof the resistorlD Vis connected `to .the amplier resonantinputcircuit through the control ,path which includes the lead designated With :alternating 4current ,voltage applied to the plat e I of the ithyratron tube 9, the plate current, over part.of :the range, :dependson fthe bias voltage ',applied. 'The sgnal tends to reduce the .effective 1bias,fmaking.the thyratronpassmore plate-curs Tent, AWhich tends to bias Y.off thelamplier and :reducezthe 'amplication Y :In '.Fig, 2, asignal voltage tends to give the same effect as a reduction of `bias on thezgridof -the-'thyratronftube jfWhen a strong signal isap- 4plied toithisgrid, the thyratron plate current will start vsooner in eachfcycle of the plate v-voltage.

"Iheldrop in theplate :resistor Wi1l'thenbe1greater, .andthebias onztheramplier tube larger.

While I have .indicated and .described several :systems .for'carrying .my invention Iinto effect, it `will 'l be apparent :to :one` skilled :in the art ithat :my :invention is .byno zmeans ylimitedgto vthe particular organizations shown and described, but that many modications may be made withoutldepart ingfi-romithescope'of mylinve'ntion asfsetlforth in the appendedfclaims. Y

What I claim is: Y :1. In combination :with :a signal `.amplifier vand a detector. a source of alternating current volt-V age, a grid controlled rectifier having its plate circuit coupled to the alternating voltage source for energization of the rectifier anode, a resistor in the anode circuit of the rectifier, direct current connections connecting the input electrodes v of said signal amplifier across said resistor, and a connection between the detector and the grid of said rectifier.

2. In combination with a signal amplifier and a detector, a source of alternating current voltage, a grid controlled rectifier having its plate circuit coupled to the alternating voltage source for energization of the rectifier anode, a resistor in the anode circuit of the rectifier, direct current connections connecting the input electrodes of said signal amplifier across said resistor, and a connection between the detector and the grid of said rectifier, said rectifier having an extremely high mutual conductance whereby the rectifier is adapted to adjust the sensitivity of the amplier, in response to variations in signal energy input to the detector, to keep the signal level of the detector grid at a substantially constant level.

3. In combination with a signal amplifier and a detector, a source of alternating current voltage, a grid controlled rectifier having its plate circuit coupled to the alternating voltage source for energization of the rectifier anode, a resistor in the anode circuit of the rectifier, direct current connections connecting the input electrodes of said signal amplifier across said resistor, and a connection between the detector andthe grid of said rectifier, the characteristics ofV said rectifier being such that a change of only a few tenths of a volt on the rectifier grid is sufficient to cause a variation in the gain of said signal amplifier.

4. In combination with a high frequency amplier and a signal rectifier, a gain control network for said amplifier, said network including a grid controlled rectifier having its grid connected to the signal rectifier circuit, means for impressing an alternating current voltage on the anode circuit of said grid controlled rectifier, a direct current connection between the anode circuit of said control network rectifier and a gain control electrode of said amplifier, a potential change of less than a volt on the grid of the control rectifier being ample to cause a change in the amplifier gam.

5. In combination with a high frequency amplifier and a signal rectifier, a gain contro! network for said amplifier, said network including a grid controlled rectifier, means for impressing an alternating current Voltage on the anode circuit of said grid controlled rectifier, a direct current connection between the anode circuit of said control network rectifierand a gain control electrode of said amplifier and a radio frequency signal connection between the control electrode of said control network rectifier and the control electrode of said rectifier.

6. In combination with ahigh frequency amplifier and a signal rectifier, a gain control network for said amplifier, said network including a grid controlled rectifier, means for impressing an alternating current voltage on the anode circuit of said grid controlled rectifier, and a direct current connection between the anode circuit of said control network rectifier and a gain control electrode of said amplifier, said first rectifier being' of the leaky condenser type, a resistor in the anode circuit of said first rectifier, and means for connecting the input electrodes of said control network rectifier across said resistor.

'7. In combination with a signal amplifier and a signal demodulator, a source of alternating current voltage, a grid controlled rectifier having its plate circuit coupled to the alternating voltage source for energization of the rectifier anode, a resistor in the space current path of the rectifier,

Y direct current connections connecting the input electrodes of said signal amplifier across said resistor, and a signal connection between the demodulator and the grid of said rectifier.

8. In a system as defined in claim 7, said source of alternating current voltage having a frequency of the order of 60 cycles.

WINFIELD R. KOCH. 

